To improve the reproduction of line images and the reproduction of continuous tone images with screen dot type images in the graphic arts field, image forming systems which have superhigh contrast (especially with a gamma value in excess of 10) photographic characteristics are required.
Known methods for obtaining high contrast photographic characteristics using stable developers involving the use of hydrazine derivatives have been disclosed, for example, in U.S. Pat. Nos. 4,224,401, 4,168,977, 4,166,742, 4,311,781, 4,272,606 and 4,211,857. Photographic characteristics of superhigh contrast and high speed can be attained using these methods, and since it is possible to add high concentrations of sulfite to the developer, the stability of the developer with respect to aerial oxidation is much better than that observed in the case of a lith developer.
However, increasing contrast with the use of these hydrazine derivatives has, in the past, required the use of a developer which has a comparatively high pH such as the pH value of at least 11.2. Because of the high pH, the developer is liable to adsorb carbon dioxide from the air resulting in the likelihood of the pH falling. Existing developers have not been sufficiently stable with respect to this aerial oxidation. Satisfactorily high contrast is not obtained when the developer pH is less than 11.2, good screen dots are not formed, and the system is unsatisfactory for the reproduction of line images.
Attempts have been made to increase the activity of hydrazine derivatives as a wa of increasing contrast at a lower pH of less than 11.2. For example, hydrazines which have substituent groups which are readily absorbed on silver halide grains have been disclosed, for example, in JP-A-60-179734, JP A-62 948, and U.S. Pat. Nos. 4,385,108, 4,269,929 and 4,243,739. (The term "JP-A" as used herein signifies an "unexamined published Japanese patent applicaiton".) Furthermore, hydrazines which undergo an intramolecular cyclization reaction in the presence of oxidized hydroquinone have been disclosed in JP-A-63-29751.
However, even these highly active hydrazines are inadequate when the pH is less than 11.2.
On the other hand, various compounds have been proposed as high contrast accelerators in JP-A-61-165752, JP-A-63-124045 and JP-A-63-133145 but although development is accelerated with these accelerators however, pH of the developer can not be reduced.
There is an additional problem in that marked changes occur in photographic performance as the pH of the developer changes. Even with slight changes in pH (e.g., fluctuation of 0.2 or more in the pH value) the area of screen dots may change and the widths of the lines in line images may change and this creates problems.